PORTABLE SYSTEMS FOR ENERGY DISPERSIVE XRAY FLUORESCENCE ANALYSIS OF WORKS OF ART
Roberto Cesareo +, Maurizio Marabelli §, Stefano Ridolfi *,
Alfredo Castellano **, Giovanni Buccolieri **, Stefano Quarta
**
, Giovanni E. Gigante ++, Antonio Brunetti +
+Istituto di Matematica e Fisica, Università di Sassari, Sassari, Italy
e-mail: cesareo@uniss.it; fax 0039-079-229482
§ Istituto Centrale del Restauro, Rome, Italy
* Ars Mensurae, Rome, Italy
** Dip. di Scienza dei Materiali, Università di Lecce, Lecce, Italy
++ Dip. di Fisica, Università di Roma “La Sapienza”, Rome, Italy
1. SUMMARY
Energy dispersive X-ray fluorescence (EDXRF) analysis is a valuable
technique for the study of works of art, because it is non destructive,
multielemental, simple and relatively inexpensive. Further, portable EDXRF
equipments can be easily assembled. For this reason EDXRF is a very
popular analytical technique in the field of "archaeometry" . Portability is,
of course, extremely useful and almost mandatory in many cases, such as
analysis of frescoes, of large paintings , bronzes, brasses and gold alloys,
and etc., especially when located in Museums . In fact, only in a few cases
it is possible to study a work of art outside its normal location (museum,
church, excavation and, etc.) and in any case, the bureaucratic problems
and high costs for doing that are prohibitive.
There are a variety of materials that can be studied by using a EDXRF
apparatus:
-paintings of all type and frescos (Table 1);
-alloys (bronzes, brasses, gold and silver alloys and so on) (Table 2);
-ceramics and porcelains, both for bulk material and decoration analysis
(Table 3);
-illuminated manuscripts;
-papers;
-stones of all type, marbles, oxidians and so on;
-glasses and etc.;
-ink.
There are then cases in which a qualitative (or semiquantitative) analysis
is sufficient (for example in the case of paintings) and others in which a
quantitative approach is required (for example in the case of alloys or
ceramics).
EDXRF analysis generally involves an area of mm2 to cm2, and a
thickness between m and fractions of mm. The analysis is, therefore,
superficial and dependent on the surface conditions .
In some case "capillary collimators" may be employed, to focus the
radiation into smaller areas of the order 10-1 to 10-4 mm2 .
1
Due to reduced involved thickness, EDXRF analysis is representative of
the bulk composition only for homogeneous samples.
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Table 1 - Elements in the most important pigments and composition 1
Antimony
Antimony white – Sb2O3
Neaple’s yellow – Pb3(SbO4)2
Lead, tin, antimony yellow – PbSnSbO 6.5
Antimony orange –Sb2S3
Arsenic
Orpiment – As2S3
Realgar – AsS
Cobalt arseniate – Co3(AsO4)2
Barium
Barium white – BaSO4
Barium yellow – BaCrO4 >1809
Bromine
Ftalocianine green – organic pigment containing Br >1930
Cadmium
Cadmium yellow – CdS >1850
Cadmium red – CdS(Se) >1909
Calcium
Chalk (Sangiovanni white) – CaCO3
Cobalt
Cobalt blue – CoO.nSnO2 >1800
Cobalt yellow – K3(Co(NO2)6) >1860
Cobalt red – CoO + MgO –modern pigment
Chromium
Chromium yellow – PbCrO4 >1800
Viridian – Cr2O3 >1860
Iron
Iron is employed in a large number of pigments:
Yellow ochre – Fe2O3 .nH2O
Red ochre Fe2O3
Prussian blue Fe4(Fe(CN)6)3
Black iron oxide – FeO . Fe2O3
Manganese
Manganese bleu – see Barium >end 1800
Terra d’ombra MnO2 + Fe2O3 +SiO2 + Al2O3 ancient pigment
Mercury
Cinnabar – HgS
Lead
Biacca – 2PbCO3 . Pb(OH)2 ancient pigment
Massicot yellow – PbO ancient pigment
Lead-Tin yellow – Pb2SnO4 >XIII Century
Minimum red – Pb3O4 ancient pigment
2
Copper
A large number of pigments is based on copper compounds, mainly bleu
and green
Azurite – 2CuCO3 . Cu(OH)2
Malachite – CuCO3 . Cu (OH)2
Verdigris – Cu(CH3COO)2 . nCu(OH)2
Selenium
Cadmium Red (see Cadmium)
Tin
Tin white >1500
Lead-Tin yellow
Lead, Tin, Antimony yellow
Titanium
Titanium white – TiO2 >1919
Zinc
Zinc white – ZnO >1840
-------------------------------------------------------------------------------------------Table 2 – Composition of copper, gold and silver alloys 2
a.bronzes : bronzes are mainly composed of copper, tin and lead ; iron is
often present as trace element ; arsenic and antimony are generally present
in oriental bronzes.
etruscan bronzes 3: Cu and Sn (5-15%) ; traces of Pb and Fe
greek bronzes: Cu and Sn (typically 10-15%); traces of Fe and Pb
roman and egyptian bronzes: Cu, Sn (typically 5-20%), Pb (typically 120%), traces of Fe, As (Egyptian).
bronzes from Mesopotamia: Cu, Sn, Pb (<1%)
bronzes from China : Cu, Sn (typically 10-30%), Pb (typically 1-15%),
traces of Fe, Ni, Zn.
b.brasses : brasses are mainly composed of copper, tin and zinc ; lead and
iron are aften present.
c. gold : Gold samples are generally composed of gold, silver and copper ;
lead and iron are also often present.
etruscan golds 4: Au, Ag (20-30%), Cu (1-2%)
micenean golds 5(Chora and Englianos excavations; 1600-1100 B.C.):
mean values: Au , Ag (20.3  7.5%), Cu (1.6 0.8%)
micenean golds of the Benaki Museum, Athens 6: Au, Ag (≈20%), Cu
(1-2%)
egyptian golds : Ag, Au, traces of Cu
celtic golds: Au (70-90), Ag (10-25), Cu (1-7)
d. silver : Silver objects are generally composed of silver, copper and lead.
Roman silver objects 7:
Coins from Alexander the Great : Ag  98% , Pb 1%, Cu,Au 1%
Roman coins "denari" : Ag  93%, Cu  6%, Au, Pb, Sn et al.  1%.
------------------------------------------------------------------------------------------
3
Table 3- Composition of typical ceramics 8 :
Main components : SiO2,(55%), TiO2,(1%) Al2O3,(20%) Fe2O3,(2%)
MgO, CaO,(2%) K2O
Trace elements : Ba (650 ppm), Rb (100 ppm), Sr (150 ppm), Y(20
ppm), Zr (200 ppm), Nb (20 ppm), V(100 ppm), Cr (100 ppm), Ni
(40 ppm), Cu (80 ppm), Zn (150 ppm) , Mn (1000 ppm).
Examples of application of portable EDXRF equipments in archaeometry
will be given, i.e.: analysis of bronzes (the statues of Perseo by Benvenuto
Cellini and of Bartolomeo Colleoni by Andrea del Verrocchio), gold alloys
(Etruscan and premicenean artefacts, and the golden altar of Sant’Ambrogio
in Milan) , silver objects and analysis of paintings (by De Chirico) and
frescos (the chapel of the Scrovegni by Giotto).
2.THEORETICAL BACKGROUND
2.1 Physical background 9
When a sample is irradiated by a beam of X-rays, secondary X-rays are
emitted , due to: a. photoelectric effect ; b. Compton effect ; elastic
scattering (Figure 1).
From the atomic point of view, when incident photons (having an
energy in the range of X-rays) interact with the atoms of a given object, the
photoelectric effect extracts an internal electron (for example from the most
internal K-shell), producing a hole in the corresponding atomic shell, which
will be filled by a more external electron (for example from the successive
L-shell), with contemporary emission of a secondary photon. The energy of
this photon is given by the binfing energy difference of the two considered
shell, i.e. EX = EK – EL (Figure 2).
In concurrence with the photoelectric effect, and with more probability at
higher X-ray energy, the incident photon may interact with an external
electron of the atomic structure, resulting deflected with respect to the
incident direction, with a reduced energy. The residual energy is transferred
to the electron. This effect is called Compton effect (Figure 3).
A further possible effect, mainly occurring at low X-ray energy, is given
by the elastic scattering of the incident X-ray photon by an atom of a given
object. Due to this effect, the incident photon is deflected, and its energy
remains constant (Figure 4).
The three above described effects finally produces a. fluorescent X-rays
of various energies ; b. Compton (inelastic) scattered photons ; elastic
scattered photons (Figure 5).
The energy of the secondary fluorescent X-rays characterizes the
elements present in the sample (see Table 4) , and the intensity is, in some
way, proportional to their concentration. Compton and elastic scattered
photons generally contributes to the background, but may be also employed
for analytical purposes
The technique based on the analysis of these secondary fluorescent Xrays is called energy-dispersive X-ray fluorescence (EDXRF). It is a non
destructive, multi-elemental and simple technique.
4
The sample can be whatever (solid, liquid, gaseous, of various size and
nature), it will in no manner be alterated by the analysis, and for that reason
it can be analyzed many times.
Further, EDXRF is a “surface analysis” because of the limited
penetration of the radiation in the sample, both primary and fluorescent. As
an example, Table 5 gives the object depth originating 90% of the
fluorescent radiation (FR), in the case of sulfur in frescoes, copper, tin and
lead in a bronze, and gold, silver and copper in a gold alloy. The values
reported in Table 4 are only indicative; the exact values, in fact, depend on
the incident energy, homogeneity of the sample, orientation of incident and
output radiation.
Figure 1 – The principle of energy-dispersive X-Ray Fluorescence analysis.
5
Figure 2 – Atomic description of the photoelectric effect.
Figure 3 – Schematic description of the Compton effect induced by X-ray
photons.
6
Figure 4 – Schematic description of elastic scattering of X-ray photons.
7
Figure 5 – Typical “spectrum” of secondary radiation emitted when a sample
is irradiated by a monoenergetic source of X-ray photons of energy E0. From
right, elastic scattering peak (of energy E0) , Compton scattering peak (of
energy EC < E0 and several fluorescent peaks are shown. In this case, E0 =
59.6 keV, EC = 48.5 keV, and Ag-K and K X-rays are shown.
8
Table 4 – Energy of X-ray lines (in keV) of the most important elements
Element
Sodium
Magnesium
Aluminium
Silicon
Phosphor
Sulphur
Chlorine
Argon
Potassium
Calcium
Titanium
Chromium
Manganese
Iron
Cobalt
Nickel
Copper
Zinc
Arsenic
Selenium
Bromine
Rubidium
Strontium
Yttrium
Zirconium
Niobium
Molibdenum
Silver
Cadmium
Tin
Antimony
Barium
Tungsten
Gold
Mercury
Lead
Uranium
K
1.04
1.25
1.5
1.74
2.0
2.3
2.62
2.96
3.3
3.7
4.5
5.4
5.9
6.4
6.93
7.5
8.05
8.64
10.54
11.22
11.92
13.39
14.16
15.0
15.8
16.6
17.5
22.2
23.2
25.3
26.4
32.2
59.3
68.8
70.8
75.0
98.4
K
L
L
2.81
3.2
3.6
4.0
4.93
5.95
6.5
7.06
7.65
8.26
8.9
9.6
11.7
12.5
13.3
15.0
15.8
16.7
17.7
18.6
19.6
24.9
26.1
28.5
29.7
36.4
67.2
78.0
80.3
84.9
111.3
1.7
1.8
1.92
2.04
2.16
2.3
3.0
3.13
3.4
3.6
4.45
8.4
9.7
10.0
10.5
13.5
2.5
3.2
3.45
3.8
4.0
5.0
9.9
11.5
11.9
12.6
17.5
All these features make EDXRF especially suitable for in-situ and on
line analysis.
A typical apparatus for EDXRF-analysis is relatively simple, and is
composed by:
1. an excitation source (a radioisotope or a X-ray tube) (Figure 6);
2. an X-ray detector with related electronics (Figure 7);
9
3. a single or a multi channel analyzer (Figure 8);
4. a dedicated software for rapid, automatic analysis of chemical elements.
In the last ten years, the technological progress has produced
thermoelectrically cooled X-ray detectors of small size and weight 10-12,
miniaturized and dedicated X-ray tubes 13-15, small size multi channel
analyzers 16,17 and dedicated softwares for quantitative evaluation 18.
Table 5 - Depth of an object irradiated with X-rays of proper energy,
giving rise to 90% of the fluorescent radiation
Object
Fluorescent radiation
of Depth involving 90%
element:
of the fluorescent
radiation
Fresco
Sulfur (2.3 keV)
20  m
Fresco
Iron (6.4 keV)
300  m
Bronze
Copper (8 keV)
55  m
Bronze
Lead (10.5 keV)
18  m
Bronze
Tin (25 keV)
120  m
Gold
Copper (8 keV)
5m
Gold
Gold (9.7 keV)
9m
Gold
Silver (22 keV)
14  m
All these progresses allowed the construction of completely portable
small size EDXRF systems with similar performance as Laboratory
systems, but without the problems connected with nitrogen cooling, big size
X-ray tubes and high costs (Figure 9).
Portable EDXRF (PEDXRF) systems are necessary in many cases,
where objects to be analyzed cannot be transported or where the mapping of
the object would require too many samples. This is particularly true in the
field of archaeometry, where samples are generally in museums, churches,
excavations and so on.
10
Figure 6 – Typical radioactive sources for EDXRF-analysis (from left to right
top:point, disc and anular sources ) and X-Ray tubes with related emitted Xray spectra : a. Ca-anode, 8kV, 0.1 mA by Hamamatsu ; b. Mo-anode, 30 kV,
0.2 mA by Oxford; c. W-anode battery-operated, 40 kV, 0.1 mA by Moxtek.
11
Figure 7 – Typical X-ray detectors and related efficiency curves. a. Si-PIN by
AMPTEK; b. Si-drift by Roentec; c. CZT by eV; HgI2 by Constellation Inc.
12
Figure 8 – Single-channel analyzer employed for analysis of Etruscan golds.
Figure 9 – A portable energy-dispersive XRF-equipment at work in St.Peter.
13
2.2 Physical principles of X-Ray Fluorescence 9
When a sample containing an element a with a concentration ca is
irradiated by a beam of X-rays having an energy E0 and intensity of N0
photons/s, the number Na of fluorescent X-rays emitted by the element a , is
approximately given by:
Na = N0 k a a ca M
(1)
where:
-k is an overall geometrical factor;
-a is the fluorescent yield of the element a in the shell of interest (i.e.
percent probability of a fluorescence effect compared with an Auger
effect));
-a (cross section in cm2) is related to the probability for fluorescent effect
of element a;
-M is a matrix term (i.e. depending on the sample) , related to the
attenuation of incident and secondary fluorescent radiation and on the
sample composition.
It is very useful to consider two extreme conditions related to the sample
thickness:
2.2.1 Thick samples 9
Artifacts like statues, columns, alloys and etc., generally appear to
EDXRF analysis as "infinitely thick samples", in the sense that the size of
the objects is infinitely large with respect to the “radiation penetration” (see
Table 2 for values of radiation penetration).
When a “thick” sample is irradiated by photons of proper energy, it emits
secondary photons which are
characteristic X-rays from the elements
composing the sample .
When a generic element a with concentration ca , in an infinitely thick
and homogeneous sample is irradiated with N0 incident photons, the
secondary fluorescent X-ray intensity Na is given by :
Na = No k a ca   ph.a (E 0 ) /  t (E 0 ) +  t (Ea )
(2)
where:
k is an overall geometric and intrinsic efficiency; a is the fluorescence
yield of element a ;  ph.a (E o ) represents the photoelectric attenuation
coefficient of element a at incident energy E o ;  t (E o ) and  t (E a )
represent the total attenuation coefficient of the sample at incident and
fluorescent energies (E 0 and E a ) respectively.
As observed above, besides fluorescent X-rays, given by Eq. (1), the Xray spectrum emitted by the irradiated “infinitely thick”sample is also
composed by scattered photons, which intensity Nsc (mainly due to
Compton scattering) is approximately given by:
Nsc  N0 k sc(E0)/ 2 t (E0)
(3)
where:
14
sc(E0) and t(E0) are the scattering and the total attenuation coefficient of
the sample at incident energy E0 respectively.
Scattered radiation is generally a disturbing effect that should be reduced
as much as possible, but it can also be employed for normalization purposes.
Equations (1) and (2) are strictly valid for thick samples and for
monoenergetic incident radiation. It may be calculated that for low values of
ca (10-15%) or for limited ca intervals, Eq. (1) yields approximately a
linear relationship between Na and ca (Figure 10) 19.
Standard samples are required for an experimental test of Eq.(1) and to
quantitatively establish the correlation between N a and c a .
Figure 10 – Correlation between counts and concentration for iron in the
situation of a thick sample.
2.2.2 Thin samples 9
In the case of thin samples, primary and secondary X-rays are
characterized by a penetration depth much larger than the sample thickness.
In this case the matrix term in Eq. (1) is approximately equal to 1, and Eq.
(1) will be given by:
Na = N0 k a a ca
(4)
i.e. counts of element a are linearly proportional to its concentration (Figure
11).
Intensity Nsc of scattered photons in the case of thin samples (mainly
due to Compton scattering) is approximately given by:
Nsc  N0 k sc(E0) m
(5)
where m (in g/cm2) is the mass per unit area of the sample.
15
Artifacts are often thin samples. For example, in the case of frescoes, (see
Section 4b), there is a thick layer of plaster over which thin layers
containing the pigments were painted (with a thickness from fractions of
mm to mm). Finally, over the pictorial layer there is, in many cases a thin
layer due to pollution effects (tenths of m) containing sulphur, typically
in the form of CaSO4 20.
For example, when this sulfur is analyzed, then the pathway of its Kradiation is comparable to the thickness of gypsum. The same occurs when
elements from Fe to Ag are analyzed: the K-radiation penetration is of the
same order of magnitude of the pigments layer. A quantitative approach is
therefore, extremely difficult in this case and generally not useful.
When radiation from a X-ray tube penetrates the pigments of a fresco or
of a painting, it is absorbed along its path. A fraction of the energy of the
absorbed photons is converted into fluorescent photons of the various
elements, and some of them, according to the thickness of the involved
layers, reach the surface of the fresco and are detected.
In the case of a fresco, the deepest layer is given by the plaster.
Superimposed there is the preparation, and above one or more pigment
layers, generally thin.
In the case of a painting, the deepest layer is given by the canvas or the
wood. Superimposed there is again the preparation, and above one or more
thin pigment layers.
Figure 11 – Correlation between counts and concentration for sulphur, in the
conditions of a thin sample.
16
As an example, in the case of Giotto’s haloes in the chapel of the
Scrovegni the complexity of the X-ray spectra puts in evidence the
presence of various pigment layers below the gold leaf. Each layer behaves
as a thin layer 21, because also elements are visible, such as strontium,
coming from the deepest layer, which corresponds to the plaster. In this
hypothesis of a sequence of thin layers, fluorescent counts Na from a generic
element a may be written in the form:
Na  N0 k a ph.a ma Ai
(6)
where:
N0 is the incident photon flux;
k is an overall geometrical and detector factor;
a is the fluorescent yield;
ph.a is the cross section of element a for photoelectric effect;
ma is the mass per unitary area of element a in the sample.
Ai gives the attenuation of incident and output radiation if element a is in the
internal layer j .
Ai is given by:
Ai = exp-1j-1 i (E0)xi  exp -j-11 (Eph.a) xi 
(7)
where:
-i (E0) and (Eph.a) is the attenuation coefficient of the i-th layer at incident
energy E0 and fluorescent energy Eph.a respectively;
-xi represents the thickness of the i-th layer
In the case of thin layers elements from the various layers are visible.
The attribution to the correct layer is in some cases possible, especially
when heavy elements are present in a deep layer, and L-lines of these
elements (gold, lead) are present and clearly visible. In these cases the
approximate thickness of the pigments may be calculated by the differential
attenuation of L-lines, and/or autoattenuation.
For example the ratio R= L /L for a heavy element, following auto
attenuation in the same element present in a pigment and having thickness x,
is given by:
R = (0+2)/(0+1)(1-exp-(0+1)x)/(1-exp-(0+2)x
(8)
where
-0 , 1, 2 are the mass attenuation coefficients (in cm2/g) at incident and
at L and L energies respectively;
- is the physical density of the sample (in g/cm3);
-x is the thickness of the sample.
Differential attenuation of L and L X-rays of a heavy element a (for
example lead) present in the second layer by another heavy element b (for
example gold present in the first layer, as in the case of gold halos which
will be discussed later) is given by:
17
L /La =  L /La0 exp-(2-1x)b
(9)
where
 L /La0 represents the L /L ratio (for example of lead) simply
autoattenuated.
The term (2-1) is positive for example for gold attenuation, because of the
gold edges, and negative for example for tin.
Figure 12 shows the attenuation coefficients of gold, lead and tin versus
energy 22, and Figure 13 the ratio R= L /L for lead L-lines attenuated
by a gold leaf or by a tin sheet.
Figure 12 – Mass attenuation coefficients of lead and gold versus energy (in
keV). Pb-L and Pb-L brackets the gold L photoelectric discontinuity and
are therefore absorbed in a different manner by different Au-thicknesses. The
Pb-L/Pb-L ratio allows, therefore, the determination of the Au-thickness.
18
Figure 13 – Ratio of L/L Pb- lines attenuated by a gold leaf, versus Au
thickness x, following Eq. 9.
A similar, but opposite phenomenon allows the determination of a tin sheet
located above a white lead pigment.
19
3.COMPONENTS OF A PORTABLE EDXRF EQUIPMENT
A portable EDXRF system is composed of a X or -ray source (a X-ray
tube or, in some cases a radioactive source), of a X-ray detector (generally a
semiconductor detector thermoelectrically cooled ) and of a multichannel
analyzer (Figure 14).
3.1 – Radiation sources
The optimal radiation source for a portable EDXRF equipment should
have the following features:
-small size and weight;
-good stability versus time;
-sufficiently flexibility in terms of energy and intensity.
a.radioactive sources
A few radioactive sources are compatible with the conditions above
cited, and can be, therefore, used in a portable EDXRF equipment. Among
them the most important are listed in Table 6 :
Radioactive sources are small and stable, and they have a fixed energy
and intensity output. However the photon flux is generally too low for
many applications, also in the field of archaeometry, and it is not flexible in
terms of energy .
b. X-ray tubes
Due to the relatively low cost of small size low power x-ray tubes,
dedicated X-ray tubes are available for each type of problem and coupled
to a specific detector 23 . They should be characterized by:
-high voltage between 5 and 40 kV approximately;
-current between 10 and 1000 A;
For low atomic number elements (from 11 to 19, including sulfur and
chlorine), a low-power Ca-anode X-ray tube may be employed 24,
working at 6-8 kV, 0.1-0.3 mA maximum voltage and current . For the
elements with Z from 11 to 17 also a low-power Pd or Ag-anode X-ray tube
may be employed, working at about 5-6 kV and hundreds of mA 23 . In
this case the L-lines of Pd or Ag are used for excitation, which energy of
2.8 to 3.2 keV is close to the excitation energy of these elements.
The last two X-ray tubes, working at 30-35 kV can be employed also for
excitation of medium (K-lines) and high (L-lines) atomic number elements.
A W-anode X-ray tube working at 40 kV, 0.1 mA may be also employed,
especially when elements from Ag to Sn should be analyzed.
20
Figure 14 – Experimental set-up of the equipment used for the analysis of
alloys (bronzes, brasses, gold, and silver). A W-anode X-ray tube is employed,
working at 35 kV, 0.3 mA, collimated with an Al-cylinder with an internal
hole of 2 mm diameter, and a Si-PIN or a Si-drift detector with about 150-200
eV energy resolution at 6.4 keV. In the Figure the equipment is shown during
the analysis of the golden altar from Volvinius.
As observed above, the photon output from the X-ray tubes is generally
collimated to irradiate an area of about 10-100 mm2 .
However, there are cases in which only a very small area or extremely
small amounts of material must be irradiated and analysed. In this case the
incident radiation can be collimated through a “capillary collimator” , and
areas as small as 10-2 to 10-4 mm2 are irradiated 25-26. The use of
capillary collimators in archaeometry will be discussed later 27 .
3.2 X-ray detectors
A X-ray detector is generally characterized by its efficiency, i.e. capacity
(in %) of processing photons entering its volume, and energy resolution, i.e.
capacity of separating X-lines contiguous in energy, expressed in terms of
energy (eV or keV).
21
A X-ray detector for a portable EDXRF equipment should further have
small size and weight.
Besides the use of traditional Si or Ge nitrogen cooled semiconductor
detectors, which are not so well compatible with portable systems, in the
last few years following small size, thermoelectrically cooled X-ray
semiconductor detectors have been increasingly employed :
a. Si-PIN, with a thickness of 300 or 500  m , an area of 6 or 10
mm2 and a typical energy resolution of about 150-200 eV at 5.9 keV
28 . This detector has an efficiency that rapidly decreases above
about 20 keV, due to the reduced thickness; the good energy resolution
of the detector is obtained with an amplifier shaping time of 12-24 s,
and therefore it deteriorates rapidly at high counting rates;
Table 6 – Radioactive sources for portable EDXRF equipments
Source Principal
Half-life
photon energies
Elements that
can be analyzed
(keV)
Fe-55
5.9
2.7 y
Z  23 (K-lines)
Cd109
22 , 88
453 d
Z  42 (K-lines)
Am241
59.5
Z =50-92 (Llines)
433 y
Z  69 (K-lines)
Z=70-92 (Llines)
b. Si-drift , with a thickness of 300  m, an area of about 4 mm2 , and an
energy resolution of approximately 140-160 eV at 5.9 keV 29 . This
detector has also an efficiency that rapidly decreases above 20 keV, but it
works much better than Si-PIN’s at high count rates, due to the low shaping
time of the amplifier (1-2 s);
c. Cd Te and Cd Zn Te (CZT) 28, with typical dimensions of 5 x 5 x
1 mm3, that have an energy resolution of about 300 eV at 5.9 keV, but
can be used in the whole X-ray energy range with good efficiency.
d. HgI2 30 , with typical dimensions of 7 x 7 x 1 mm3 , that have an
energy resolution of about 180 eV at 5.9 keV, and can also be
employed in the whole X-ray energy range.
22
3.3 Multi-channel analyzer
Due to the small size and flexibility of modern multi-channel analyzers,
they can be easily coupled to sources and detectors to generate a portable
system. Multichannel analyzers are also generally equipped with software
for element identification, background subtraction, peak evaluation and etc.
3.4 Capillary collimators 25
X-ray Optics based on polycapillary consists of an array of a large
number of small hollow glass tubes formed into a certain shape. The optic
collects X-rays that emerge from an X-ray source within a large solid angle
and redirects them by multiple external total reflections to form either a
focused beam or a parallel beam. Small spots of 20-50 µm diameter can be
obtained.
The use of polycapillary optics has become widespread in various X-ray
analysis applications, and also in the field of archaeometry, where it is often
useful to strongly collimate the incident beam, in order to carry out
“microanalysis”. The rapid development of capillary optics also triggered
the development of related X-ray equipment such as microfocus X-ray
sources.
Monocapillary optics (single tapered channel optics) also offer further
improvement in space resolution (spots of 5-25 µm diameter) beyond that
currently achieved with polycapillary optics.
Capillary collimators are shown in Figure 15, and their effects on a
fluorescent X-ray spectrum is shown in Figure 16.
4. EXPERIMENTAL SET-UP
For analysis of low atomic number elements (sulphur, chlorine,
potassium, calcium) a portable EDXRF-equipment was assembled,
composed of a Ca-anode X-ray tube working at 5-8 kV 31 (Figure 17).
In this case the incident radiation is composed of the Ca-K lines at 3.7
keV, and of the bremsstrahlung radiation.
Alternatively, also a Pd-anode X-ray tube was employed, working at 4-6
kV 28. In this case the incident radiation is composed of the Pd-L lines at
2.9 keV and of the bremsstrahlung radiation.
In both cases an AMPTEK thermoelectrically cooled Si-PIN detector
was employed, having an energy resolution of 200 eV at 5.9 keV and
coupled to a pocket AMPTEK multi channel analyser 18.
For analysis of pigments, both of Giotto’s Chapel of the Scrovegni, and
De Chirico paintings 32 , a portable EDXRF- equipment was employed,
composed of a small size, low weight W-anode Oxford X-ray tube,
working at 30 kV and 10-50 A 13, a small size, thermoelectrically cooled
AMPTEK Si-PIN detector with an energy resolution of 200 eV at 5.9 keV
28 and a pocket AMPTEK multi channel analyser (Figure 18).
23
Figure 15 – Typical capillary collimators manufactured by IfG, Berlin,
Germany.
Figure 16 – Effects of capillary collimators as shown in Figure 15, on X-ray
spectra
24
Figure 17 – PEDXRF-equipment employed for analysis of sulphur and
chlorine in the fresco of the church of S. Stefano Rotondo in Rome.
For analysis of the 14 true De Chirico paintings, a thermoelectrically
cooled Si-drift detector was employed, having an energy resolution of
about 140 eV at 5.9 keV 29 (Figure 18). The same equipment was
employed for the analysis of the equestrian statue of Bartolomeo Colleoni.
Figure 18 – Equipment typically employed for analysis of paintings.
25
In all these cases both X-ray tube and detector are placed at a distance of
5-10 mm from the fresco, at an angle of about 30° with respect to the
painting normal. An area of approximately 3-5 mm2 is irradiated and
analysed, and a typical measuring time of about 100-200 s was employed.
5.RESULTS
In the following are reported some selected results obtained in the field of
archaeometry, in the case of bronzes, paintings and frescos and gold
artefacts.
5.1.Bronzes:
The Perseo by Benvenuto Cellini 33
The huge statue of Perseo by Benvenuto Cellini (completed in 1554 and
located in Piazza della Signoria in Florence), composed of 1800 Kg of
bronze was analyzed during the restoration process in the Uffizi Museum in
Florence . The Perseo (figure 19) is composed of two parts: the Perseo
itself and the Medusa ; additionally some accessories were analyzed, such
as the sword.
These bronzes are covered by a thick patina, and therefore 10 areas of
about 5x5 mm2 were cleaned before the analysis. In addition, 4 microsamples were withdrawn and 2 metallographic samples.
All these samples were analyzed by EDXRF with metallographic and
electric conducibility studies performed as well 34 . The results were
generally consistent. Typical X-ray spectra of Perseo and Medusa are shown
in the same Figure 18. The mean values of Perseo and Medusa bronzes are
shown in Table 7.
Table 7- Mean concentration values (in %) of Perseo and Medusa
bronzes
Object
Perseo
Cu
Fe
94  1.5 0.5  0.3
90  2
0.4  0.3
Pb
2.2  0.3
1.2  0.3
Ag
0.05  0.1
0.05  0.1
Sn
2.5  0.3
7.0  0.3
Sb
0.6  0.3
0.4  0.2
Medusa
From the results of Table 3 it appears clear that Perseo and Medusa have
different compositions.
Following Benvenuto Cellini , due to problems during the casting of the
Perseo he threw in the fusion 60 pounds of Sn and 22 english dishes (which
contain about 10% Sb).This could be compatible with the results of Table 7.
26
Figure 19 – The famous bronze statue "Perseo" by Benvenuto Cellini, Piazza
degli Uffizi, Florence, and two tpical X-ray spectra of Perseo and of the
Medusa head respectively, showing the different (but similar) composition of
the two parts of the huge statue.
27
The equestrian statue of Bartolomeo Colleoni by Andrea del
Verrocchio, campo SS. Giovanni and Paolo, Venice.
The huge equestrian statue of Bartolomeo Colleoni (Figure 20) was
created (but not terminated) by Andrea del Verrocchio around 1480 35.
It is a gilded bronze having a height of about 4 meters (without base) and a
weight of about 4000 Kg. It is located in Campo SS. Giovanni e Paolo in
Venice, and was under restoration when the measurements were carried
out..
Figure 20 – The huge equestrian statue of Bartolomeo Colleoni by Andrea del
Verrocchio (about 1480) , located in Campo SS. Giovanni e Paolo in Venice.
28
The statue was systematically analyzed in 21 points during its restoration
(see Table 8) , with the following purposes:
a. determine the presence of sulphur and chlorine due to pollution and
influence of the sea water;
b analyze the patina composition and thickness;
c. determine the composition of the horse and the knight alloys;
d. determine the composition of the soldering areas (3,6,7,9,12,13,15).
To this aim 21 points were selected, and first analyzed without any cleaning
of the surface, in order to clarify points a. and b. Further, an area of few
mm2 of these 21 points were completely cleaned to remove patina and
analyzed.
Following results were obtained:
a.sulphur and chlorine are present almost everywhere on the surface of
the statue (see Table 8). It is not easy to exactly determine their
concentration but it should range in the order of percents to tens percent;
a typical X-ray spectrum showing the presence of S and Cl is shown in
Figure 21;
Figure 21 – Sulfur and chlorine in area 7 (horse, left side of the belly,
soldering area, not exposed to rain) of the equestrian statue of Bartolomeo
Colleoni (see Table 8). This X-ray spectrum was obtained with the equipment
composed of the 40 kV W-anode X-ray tube (working at 5 kV) and Si-drift
detector. The patina has a thickness of about 150 µm.
29
b.The behaviour of the patina composition versus “bulk” composition
strongly depends on the exposition of the analyzed area, i.e. if it was
exposed to rain, sun, humidity and so on. The patina contains much more
tin, lead and antimony, and less copper and zinc in the areas strongly
washed out (see for example areas 1,2,9,15,17,18,19 and 20), whilst the
composition of the patina is similar to that of the cleaned surface when the
area is protected (see for example areas 7, 10 and 11) ;
c.Typical X-ray spectra of area 10 (before cleaning at 7.5 keV and 35 keV
incident energies respectively and at 35 kV after removing the patina) are
shown in Figure 22. Areas 1, 2 and 5 (posterior legs of the horse) show –
with some doubt for area 5- a similar composition; the same for areas
8,10,11 and 14 (the two anterior legs of the horse and its neck),and for areas
18,19 and 20 (arm and chest of the knight) . The welding areas 3,6,7,12,13
and 15 show a similar content of tin and lead (but different in zinc) ;
However these last areas are not completely reliable, because the EDXRF
apparatus was hold by the hand of the operator. The conclusion is that the
anterior part of the horse has the same composition; the posterior legs have
also possibly the same composition. The queue was superimposed at a
different time. Concerning the knight, the superior part has a similar
composition. The left leg seems to be a different composition, so as the
helmet, which has a quite different composition. However in area 17 tin
content is strongly dependent on the cleaning, and cannot be excluded that a
small part of the irradiated area could be not enough cleaned. The same
consideration could be made for areas 1 and 2. (Table 8).
30
Figure 22 – X-ray spectra of area 10 of the equestrian statue of Bartolomeo
Colleoni before removing patina (spectrum A at 7.5 kV and spectrum B at 35
kV), and after removing the patina (spectrum C at 35 kV) (see Table 8).
31
Further general considerations are the following:
-Iron is almost constrant in all areas of the statue, with a mean value of
0.29±0.07%; the only exception is given by area 13, where the
concentration of iron is 6 times higher;
-The ratio Sb/Sn is quite constant in all areas, except for areas 20 and 21;
-The copper concentration is not too much varying (83.3±4.7 for the two
statues; 84.3±4 for the horse and 80±5 for the knight)
a.Soldered areas which are junctions between the various parts of the horse
(areas 3,6,7,9,12,13 and 15) are basically brasses, and in some cases they
have a composition similar to that of the neighborough areas, with tin
substituted with zinc (see for example areas 5 and 6, and areas 11,12 and 13.
An exeption is given by area 9, in which a small quantity of zinc (3.4%)
seems to have been added to the bronze of the neighboroughts areas 8 and
10.
Table 8 – Analysis of the equestrian bronze statue of Bartolomeo
Colleoni
Area
n.
1
1’
2
2’
3
3’
4
4’
5
5’
6
6’
7
7’
8
8’
9
9’
10
10’
11
11’
Cu
(%)
73.3
22.8
78.1
41.5
84.0
84.1
67.6
71.9
84.6
78.6
88.4
84.4
88.0
89.7
84.0
69.6
81.4
41.4
84.9
80.1
86.5
80.4
Sn
(%)
20.2
64.7
17.1
42.8
2.3
5.3
24.6
22.7
11.9
16.2
4.1
7.7
0.5
0.5
12.0
22.5
12.0
43.6
11.3
14.8
10.2
14.6
Pb
(%)
4.0
6.0
2.6
9.3
2.7
5.9
4.9
1.6
2.0
2.3
1.4
2.1
2.5
2.5
1.9
4.2
1.9
8.8
1.9
1.8
1.8
1.5
Sb
(%)
2.1
5.9
1.8
5.3
0.25
0.6
2.5
2.6
1.2
1.9
0.6
1.3
<0.05
0.4
1.5
2.5
1.0
3.9
1.6
2.2
1.3
2.1
Ag
(%)
0.08
0.1
0.08
0.5
<0.01
0.1
0.1
0.1
0.02
0.1
0.02
0.1
<0.01
0.1
0.05
0.1
0.03
0.5
0.03
0.1
0.03
0.1
Fe
(%)
0.35
0.5
0.3
0.6
0.3
0.8
0.3
1.2
0.3
1.0
0.4
1.5
0.45
1.0
0.3
1.1
0.3
0.9
0.25
1.1
0.2
1.3
Zn
(%)
0
0
0
0
10.4
3.2
0
0
0
0
5.1
3.0
8.5
5.9
0
0
3.4
0.9
0
0
0
0
32
12
12’
13
13’
14
14’
15
15’
16
16’
17
17’
18
18’
19
19’
20
20’
21
21’
83.9
90.0
86.9
84.9
87.1
81.1
89.8
79.3
87.8
82.1
73.8
35.7
80.4
38.1
81.1
41.2
79.9
57.0
<57.0
35.1
0.9
1.4
0.9
3.2
9.1
12.4
4.4
11.5
8.9
13.1
21.2
49.8
15.0
45.8
14.8
44.2
12.7
26.7
36.6
52.2
1.6
4.1
2.9
5.5
1.8
2,5
1.6
5.0
1.6
2.1
1.7
5.7
2.6
9.7
2.2
8.4
1.6
5.3
3.8
7.3
<0.05
0?
0.15
0.5
1.8
2.6
0.8
2.3
1.4
1.4
3.0
7.8
1.8
5.6
1.6
4.6
5.5
9.6
2.6
4.4
0.02
0?
0.03
0?
0.03
0.1
0.02
0.1
0.03
0.1
0.04
0.5
0.04
0.6
0.02
0.5
0.07
0.6
LS
0.5
0.35
1.5
1.7
3.2
0.2
1.4
0.35
0.9
0.3
1.1
0.25
0.6
0.2
0.3
0.25
1.1
0.25
0.8
LS
0.5
13.2
3.0
7.4
2.7
0
0
3.0
1.0
0
0
0
0
0
0
0
0
0
0
0
0
Dark numbers without apex are related to measurements on cleaned areas
1. horse, right posterior leg, area exposed to rain;
2. horse, right posterior haunch, area exposed to rain
3. horse, right posterior haunch, soldering area
4. horse,queue, area exposed to rain
5. horse, left posterior leg, area exposed to rain
6. horse, laft posterior leg, under welding; area partially exposed to rain
7. horse,left side of the belly, soldering area, not exposed to rain
8. horse,left anterior leg, area exposed to rain
9. horse, left anterior leg in soldering area, area exposed to rain
10.horse, left anterior leg, area not too much exposed to rain
11.horse, right anterior leg, internal area under soldering, not exposed to
rain
12.horse, right anterior leg, soldering area
13.horse, right anterior leg, other soldering area
14.horse, neck, over soldering, area exposed to rain
15.horse, soldering area between neck and body, area exposed to rain
16.knight, left calf, area exposed to rain
17.knight, dress in area of left haunch, exposed to rain
18.knight, left hand, area exposed to rain
19.knight, left forearm
20.knight, chest in area close to the hand
21.knight, helmet, exposed to rain (measurement not reliable; equipment in
precarious position.
33
Etruscan bronzes 3
Etruscan bronzes are mainly Cu-Sn alloys, with an erratic presence of
lead. Examples of analysis of Etruscan bronzes are given in Table 9.
Table 9 – EDXRF-analysis of Etruscan bronzes (concentrations in
percent)
Object
Jamb from Granmichele;
IX Century BC
Phial from Poggioreale;
VIII Censtury BC
Chariot from Populonia
Chariot from Ischia di
Castro; VI Century BC
Vase from Tuscanica; IV
Century BC
Chariot from Chianciano
IV Century BC
Canopo from Dolciano;
VII Century BC
Charioteer ; V Century
BC
Jug from Lecco; V
Century BC
Chimera from Arezzo; V
Century BC
Cu
90.5
Sn
8.9
Pb
0.6
89.8
9.6
0.6
90.0
81.4
9.8
18.0
0.2
traces
90.0
9.1
0
84.0
12.4
1.0
97
2.9
0.2
86.1
13.3
0.6
88.6
10.5
traces
82
15
3
Bronze equestrian statue of Marco Aurelio 36
The bronze equestrian statue of Marco Aurelio (Figure 23), located in
“piazza del Campidoglio” , Rome, was analyzed in a large number of areas,
by removing microfragments. Results are shown in Table 10.
Table 10 – EDXRF – analysis of the statue of Marco Aurelio
N. and sampling area
Cu
(%)
Sn(%)
Pb(%)
1-left hind leg –shin
80
10.4
9.6
2-left hind leg-forearm
80
9
19
3-right fore leg, shin
82
8.4
9.5
4-right fore leg - arm
80
8.3
12
34
15-belly, right side
79
7.3
13.4
32-belly, right side
78.4
5.3
16.3
47-belly, right side
82
6
12
35-belly, left side
81.5
4.4
14
46-belly, left side
79
7
13.6
28-left chest
80
6.7
13
31c-left chest
78
7.8
13.5
58-head, left side
79
8.2
12.5
42-knight,right knee
82.4
7
10
43-knight, right thigh
84
7.2
8.4
44-knight, cloak
82.4
6
11.6
5.2.Frescos
Sulfur in frescoes
Superficial sulfur, in the form of Ca S O4 (gypsum), is an index of
pollution. It is often present on the surface of frescoes and monuments,
producing black coloring and damages (20). Sulfur and chlorine were
analyzed with the apparatus as described in Section 2.
By using the Ca-anode X-ray tube the minimum detection limit for
sulfur is 0.1% in 100 s measuring time, at 3SD from the background. By
using the Pd-X ray tube the MDL is approximately the same. When both S
and Cl are present, then the Ca-anode apparatus is much better for Cl,
because the separation between Cl-lines and exciting peak is greater in the
first case.
The following frescoes were analyzed :
a. frescoes attributed to Pomarancio in the church of S. Stefano Rotondo in
Rome, which was under restoration by the “Istituto Centrale del Restauro”
of Rome ( see Figure 17). A large number of areas were analyzed 37
and three typical situations were detected : a. in unrestored areas, sulfur was
found everywhere at concentrations up to about 12%; b. areas that were
simply sponged with a proper solution, sulfur was found at concentrations
between 2% and 4% ; c. in areas accurately treated to remove pollution
layers, no sulfur was found.
b. frescoes of Piero della Francesca, Church of S. Francesco, Arezzo, where
sulfur was found practically everywhere 37 ;
c. frescoes of Domenichino, Nolfi Chapel, Cathedral of Fano, again sulfur
was found everywhere, except in the restored areas 37 .
d.ancient roman frescoes, Church of S. Clemente, Rome . In this church
both the ancient fresco in the mithraic school and the fresco in the lower
basilica were analysed 37 .
35
Figure 23 – The equestrian bronze statue of Marco Aurelio, piazza del
Campidoglio, Rome, Italy.
36
Large quantities of sulfur were detected in the lower basilica, at the
upper level, close to the outside air. No sulfur was detected in the mithraic
school, which is underground, in an isolated location.
e. the famous frescoes by Giotto in the "chapel of the Scrovegni" in Padua
were recently systematically analysed in about 300 points, before and during
restoration, in order to detect the possible presence of sulfur and to
characterize the pigments composition employed by Giotto.
Begun in 1303 and consecrated on march 25, 1305, the chapel, dedicated
to Our Lady of the Annunciation, was commissioned by Enrico Scrovegni
in suffrage for the soul of his father, Reginaldo, accused of usury. It was E.
Scrovegni who commissioned Giotto to execute the frescoes in the interior
of the chapel, where the Master attained the height of his artistry, for this
cycle of paintings signals " a point of no return in the entire history of
western painting" (Figure 24).
Sulphur was analyzed with two different types of equipment: one using
the Ca-anode X-ray tube, the second one using the Pd-anode working at low
voltages, to selectively excite Pd-L lines, with an energy of about 2.8 keV,
suited to the excitation of sulfur and chlorine. The fresco-pigments were
analyzed with the same Pd X-ray tube working at about 10 kV, and with a
W-X ray tube working at 30 keV.
The following results were obtained 38:
-Sulfur was detected everywhere, at a concentration level from about 1% to
about 10%, depending on the exposition and on the undergoing pigment;
sulfur content was for example lower in the case of azzurrite pigments,
higher in the white and green pigments; the S-content strongly decreases
after using a cleaning process based on ion-exchange resins (Figure 25); the
use of the Ca-anode X-ray tube gives rise to a "cleaner" spectrum with
respect to the Pd-L X-ray tube, but the counting rates are much lower, due
to the large output window of the first tube (X-ray tubes output is strongly
collimated to irradiate an area of about 1 cm2);
analysis of Giotto’s haloes in the Chapel of the Scrovegni
About 30 haloes were analysed, many of them in good conditions (golden
haloes), other damaged, and other completely black 21-22.
A X-ray spectrum of a good condition gold halo compared with the Xray spectrum of a black one is shown in Figure 26. From left to right
fluorescence peaks are visible due to the following elements:
-gold M-lines at 2.1 keV;
-sulphur K-lines at 2.3 keV, due to pollution effects;
-lead M-lines at 2.34 keV;
-argon K-lines, at 2.95 keV, due to the presence of this element in air;
-tin L lines, at 3.45 keV, present in the black halo only;
-calcium K-lines, at 3.7 keV;
-iron K and K lines, at 6.4 and 7.06 keV;
-nickel K and K lines, at 7.5 and 8.3 keV, due to background effects in the
X-ray tube;
-copper K and K lines, at 8.04 and 8.94 keV;
37
-tungsten L-lines, at 8.35, 9.8 and 11.3 keV respectively, due to the X-ray
tube anode;
-gold L-lines, at 9.67, 11.5 and 13.4 keV, present in the golden halo only;
-silver K-lines, at 22.1 and 25.2 keV, mainly due to fluorescence effects in
the detector;
-lead L-lines, at 10.5, 12.6 and 14.8 keV;
-strontium K lines, at 14.15 keV;
-tin K and K lines, at 25.2 and 28.7 keV respectively, present in the black
halo only.
Figure 24 – General view of the Chapel of the Scrovegni in Padua, painted by
Giotto between 1303 and 1305. Specifically shown is the “life of Christ”.
38
Figure 25 – Results of sulfur cleaning procedure of Giotto’s fresco in the
chapel of the Scrovegni in Padua.
39
Figure 26 – Comparison between X-ray spectra of a golden halo (left), a black
halo in which the gold leaf is absent (middle), and a black halo with a tin sheet
at the surface, instead of gold. The different ratios of L/L Pb-lines (Pb is
present under the halo, as preparation) in the three X-ray spectra is due to
different attenuation of Pb-L lines by the gold leaf, auto-attenuation in Pb,
and attenuation by the tin sheet respectively. The Pb-L line is more
attenuated by the Au leaf than the L line; the opposite occurs with the black
halo having tin at the surface.
40
There are several cases of peaks overlap: sulphur K with lead M, tin L
with calcium K, gold L with tungsten L.
X-rays of elements argon, nickel, tungsten and silver are due to the Xray tube anode (W), or to the interaction of the X-ray beam with the detector
(Ag), air (Ar), tube material (Ni) , the other X- lines are related to the fresco
pigments and/or to the plaster. However, they must be assigned to the
proper layer.
The ratio of the X-rays of all elements with respect to gold L-X rays was
first calculated and the Pb (L/L) ratio (Table 11). If an element belongs
to the gold alloy, typically composed of Au, Ag, Cu, Pb and Fe, then its
ratio with respect to gold should remain approximately constant.
From the mean values of these ratios it may be deduced that no one of
the elements essentially belongs to the gold alloy, even if cannot be
excluded that a small amount of them could belong to it. The gold employed
by Giotto is, therefore, with high probability, of high purity.
Further, lead should belong to the “second layer”, because it appears at
the surface when gold is partially damaged. This point is confirmed from
results shown in Table 11, where it may be observed that lead/gold is not
varying too much. In this hypothesis, of a gold leaf superimposed to a layer
of lead white, the Pb-L lines should be attenuated in a different manner by
the gold leaf, when present. This effect is, in fact, clearly visible in Figure
26, where the X-ray spectrum of a golden halo is compared with a black
one, in which the contemporary presence of tin and lead is apparent (in this
case a tin sheet is superimposed to the white lead pigment). The different
attenuation of Pb-L and Pb-L lines by gold and tin is clearly visible.
By plotting the attenuation coefficients of gold, lead and tin (Figure 12),
it may be calculated that Pb-L lines are more attenuated with respect to PbL lines when crossing a gold leaf, less attenuated when crossing a tin sheet
(Figure 12).
Considering these effects for all gold haloes in good conditions, the mean
thickness of the gold layer may be calculated, which turns out to be:
1.6 0.5 m
From this result it may be concluded that the gold leaf is extremely thin
and of relatively constant thickness (minimum and maximum values: 1 m
and 2.3 m respectively). Calculating the total area covered by gold haloes,
the total amount of gold employed by Giotto can be approximately
evaluated as mAu = 540  170 g .
The thickness of the layer of white lead (basic carbonate of lead) can be
calculated from the Pb-L lines autoattenuation when the Au-leaf is no
present as being about (62)m Pb-equivalent, corresponding, of course,
to a much larger thickness of the pigment.
Complicated is the attribution of copper to the correct layer. Looking at
the X-ray spectra of irradiated areas, it turns out that X-rays of Cu are
clearly more intense when the halo is superimposed to an azurite
background, which is at a deeper layer than lead . Excluding these cases,
from Table 11, Cu continues to present and the calculated ratio Cu/Au is
approximately constant at the value Cu/Au  (0.6  0.25). It is therefore
41
reasonable that Cu-X rays come both from the azurite and from a layer
below the gold leaf. From other measurements and considerations it seems
probable to be between lead carbonate and gold (may be Cu-resinate
employed to glue the gold leaf on the white lead preparation). From the
ratio Cu/Au  0.6 it turns out that the copper equivalent thickness of the
glue between lead and gold is of about (0.90.3)m.
Considering now the Cu-K lines from the azurite layer, which can be
identified by the much higher intensity, they are attenuated by the lead+gold
sheet, but the K line is more attenuated, giving rise to a K/K ratio of
about 8.5 instead of the “normal” value of 6.4. This effect can be observed
in a few X-ray spectra where the Cu-lines are sufficiently clean. The Cuequivalent thickness of azurite can be calculated as being about 5 m.
Calcium, iron and strontium could come, at least theoretically, from the
deepest layer: the plaster. In this hypothesis Ca, Fe and Sr-K lines should be
attenuated by the superimposed sheets of lead carbonate, copper and gold,
giving rise to an attenuation factor of about 106, 35 and 3 respectively. In
the case of Ca this attenuation seems to be too high to give reasonable Cacounts in the X-ray spectra. Further, Ca-K line should be completely
absorbed by the lead+gold layers, and this effect was never observed. Ca
should be, therefore, also present at the surface of the fresco, possibly as
CaSO4 . This hypothesis is confirmed by X-ray spectra obtained with a Xray tube working at 5 kV, where the penetration of incident radiation is
extremely reduced. In those spectra large peaks of sulphur and calcium are
present.
Also the attenuation factor for Fe seems to be too high, and the ratio of
K/K, in the few cases in which it could be calculated, seems to be not
compatible with the hypothesis of Fe coming only from the plaster. May be
iron is also present in a more superficial not exactly identified layer.
Strontium is a minor component of the plaster. In fact the peak of this
element is present in almost all X-ray spectra of the fresco, at higher levels
when Fe or Cu pigments are superimposed to the plaster, and at lower levels
in the case of golden haloes, when the Sr-peaks cross Pb+Au, or Sn+Pb
layers.
A possible reconstruction of the various involved layers is shown in
Figure 27.
Seven haloes are black, and contain high quantities of lead, but no gold.
Besides that, the X-ray spectra are quite similar to those of golden haloes.
The ratio Pb-L/Pb-L is  1.57, which corresponds to a Pb-L/Pb-L ratio
affected by auto attenuation only. The golden leaf was possibly lost.
Two additional haloes are black and also similar, and contain high
quantities of both lead and tin. Besides lead and tin the X-ray spectra are
similar to those of golden haloes. The ratio Pb-L/Pb-L is about 1.1,
corresponding to the situation of a tin layer superimposed to one of white
lead. The thickness of tin, calculated from the Pb-L/Pb-L ratio turns out to
be about 10 m.
A possible reconstruction of the various layers involved in this case is
shown in Figure 28.
42
Table 11 – Pb(L/L) ratio and ratio of the intensity of elemental X-rays
with respect to gold intensity for golden haloes.
Sample
n.
245
248
294
302
207
208
209
346
227
228
348
236
238
239
240
242
246
295
296
310
319
320
246
Mean
values
Pb(L/L)
Pb-L/Au-L Fe/Au
Cu/Au
1.78
1.9
1.71
1.67
1.85
1.85
1.87
1.80
1.80
1.67
1.74
1.65
1.65
1.77
1.66
1.70
1.65
1.77
1.65
1.74
1.8
1.79
1.71
1.750.08
4.2
11.8
6.4
7.1
3.7
2.7
3.9
3.4
3.5
4.4
3.4
7.4
10.4
6.9
6.4
4.2
3.8
6.0
6.5
6.7
8.0
3.6
3.9
5.62.2
0.6
2.4
2.2
1.8
2.0
7.4
0.9
0.7
0.7
0.8
0.6
1.1
0.6
0.5
0.5
0.3
2.4
1.2
1.3
1.5
0.18
0.2
1.7
1.31.4
2.2
1.0
3.9
1.8
5.0
6.5
12.4
4.4
3.4
8.0
3.7
3.3
4.7
2.6
1.9
2.1
2.4
4.8
6.4
2.2
2.5
3.9
5.2
4.13.0
43
Figure 27 – Possible “reconstruction” of the various layers present in a
golden halo (left). An example of the X-ray spectrum of this halo is also
shown.
44
Figure 28 – Possible “reconstruction” of the various layers present in a
black halo (left). An example of the X-ray spectrum of this halo is also
shown.
45
Paintings by Giorgio De Chirico
analysis of 15 paintings of De Chirico
15 paintings of the last period of Giorgio De Chirico (1960-1970) were
analysed, to identify the pigments typically employed by the artist in this
period 32. All these paintings seem to have a similar composition. The
“pingerprint” of these paintings is:
-a preparation made with a mixture of lead white and zinc white;
-a systematic use of lead, not only for the preparation;
-the red colours based on the use of cinnabar (HgS);
-a moderate use of organic pigments.
A typical painting with corresponding analysed points and X-ray spectra is
shown in Figure 29.
analysis of 11 paintings supposed of De Chirico
11 paintings supposed of De Chirico were analysed, to identify the pigments
typically used by the author (authors) and to establish if these pigments are
similar or different from those employed by De Chirico 32. First of all it
was verified that all paintings seem to have a similar composition,
signifying that they were painted by the same artist. The “fingerprint” of
these paintings is:
-a preparation made with zinc oxide;
-almost absence of lead;
-the red colour made with cadmium red;
-a very frequent use of organic pigments.
A typical painting of this type, showing the analysed points and related Xray spectra is shown in Figure 30.
X-ray spectra of a quite similar true and supposed De Chirico painting
are shown in Figure 31.
They appear clearly made by different artists. Comparing all the results on
certain and supposed De Chirico paintings, they seem to be very different.
The 11 paintings supposed of De Chirico are, therefore, with high
probability fakes.
46
Figure 29 – X-Ray spectrum of a De Chirico painting, Fondazione De Chirico
collection, Piazza di Spagna, Rome.
47
Figure 30 – Self portrait of De Chirico, Fondazione De Chirico collection,
Piazza di Spagna, Rome, and X-ray spectrum of a red pigment.
48
Figure 31 – portrait of De Chirico, supposed to be painted by the artist, and
X-ray spectrum of a red pigment.
49
5.3.Gold artifacts
A large number of etruscan gold objects from the VII Century B.C. has
been analyzed, from the Vatican Museum and Villa Giulia Museum in
Rome, and from the Museum of Tarquinia. Selected results are collected in
Table 12.
Table 12-Analysis of selected Etruscan gold objects from the VII
Century B.C. 4
50
The golden altar of Sant’Ambrogio
The golden altar of S. Ambrogio in Milan is considered one of the most
important goldsmith’s work ever realized 39. It was constructed
approximately in the period between 824 and 859 A.C. from Volvinius, not
historically identified . It is composed of four sides, three –north, south and
east- made on gilded silver (but gold survived only in a few areas) and one –
the west side- on gold. Each side is composed of 12 panels.
Ancient gold is generally composed of gold, silver and copper, while
ancient silver is normally composed by copper, lead, gold and iron (gold
and iron at concentrations below 1%) and in some cases tin, nickel and
zinc at trace levels 2. In silver alloys, there is a frequent occurrence of
surface enrichment, especially for base silver or for copper-rich alloys .
The altar of Volvinius was analyzed in about 200 areas . Mean values of
the results are shown in Table 13 40.
Table 13 – Summary of the EDXRF-measurements on the altar of
Volvinius 40
NORTH
SIDE
Silver
Gildings *
SOUTH
SIDE
Silver
Gildings *
WEST SIDE
(Figure 15)
Gold
20th century
panels
EAST SIDE
(Figure 16)
Silver
Gildings *
Fe (%)
Cu (%)
Au (%)
Ag (%)
0.4  0.3
0.1
3.1  1.8
0
1.8  0.4
85
94.7  1.7
15
2.9  1.8
0.1
1.8  0.4
87.6
94.8  2.2
12
0.6  0.65
0
3.8  2.5
0
93.5  2.5
98.5
2.1  1.7
1.5
0.32  0.25
0.23
4.1  0.9
0
1.3  0.5
88.8
94.3  1.4
11
0.38
0.3
 0.3
*Only mean values without errors are given because the gilding concentration values are
subject to large fluctuations, due to its thin and variable depth (10-15 m) in all areas.
From the above results the following conclusions can be made:
-In the three silver sides, silver has the same composition, i.e. (Ag  94.6% ,
Au  1.6%, Cu  3.4%, Fe  0.4%); remarkable is the presence of gold in
the silver alloys (Figure 32) ; it could depend on residual (not visible)
guilding residues;
-Gilding in the silver sides seem to have a similar mean composition, i.e., Fe
 0.2%, Cu = 0, Ag  12.7%, Au  87.1% in spite of the complications
due to its reduced thickness;
51
-In the gold panel, gold has the mean composition : Au  93.5 % , Ag 
2.1%, Cu  3.8%, Fe  0.6% (Figure 33), except in the 3 panels of the 20th
century ;
-The external frames have a very variable composition; most likely because
they were all remade.
Figure 32 – East side of the altar of Volvinius, on silver with guildings, and a
typical X-ray spectrum of this panel, obtained with a W-anode X-ray tube
working at 30 kV and 0.2 mA, and a Si-PIN detector.
52
Figure 33 – West side of the altar of Volvinius, on gold, and a typical X-ray
spctrum of this panel, obtained with a W-anode X-ray tube working
at 30 kV and 0.2 mA, and a Si-PIN detector.
53
micenean and pre-micenean golds 6.
Several gold objects of micenean and pre-micenean period were
analyzed in Athens, at the Benaki Museum . As an example, two bulky
vases from Eubea were analyzed, quite similar and both dated arout 2000
B.C.(figure 34).The results are shown in Table 14
Table 14- EDXRF-results (in %) of two eubean gold vases ( 2000 B.C.)
Object
Vase .2049
(Fig.19)
Vase n. 1516
(Fig. 19)
Au
80.5
Ag
18  2
Cu
1.4  0.3
Fe
0.05  0.1
79.5
21  3
0.9  0.3
0.05  0.1
The two vases of same origin and age seem to have also a quite similar
composition.
A quite different composition was found in a thin decorated vase , which
was analyzed at 8 different places. The results are shown in Table 15 .
Table 15 - EDXRF results (in %) of a thin decorated vase
Vase
27515
1
Au
Ag
K(*)
Ca (*)
Cu
Fe
98.9
0
0.3
0.3  0.1 0.5
2
(Figure
20)
3
4
5
6
7
96.6
0.2*
*
0.2
0.5  0.2
5.5  0.2
0.2
2.5
98.2
97.7
97.0
95.1
98.5
0.2
0.2
0.2
0.2
0.2
0.3
0.4
0.5
0.5
0.1
1.2
2.8
3.2
5.3
1.2
0.3
0.3
0.3
0.2
0.3
1.2
1.7
1.9
4.0
0.9
8
(Figure
20)
96.5
0.2
0.5
5.0
0.3
2.8
(*) possibly due to burying.
(**) indirectly determined as 3 SD from the background under the Ag-k
line.
54
Figure 34 – Two eubean gold vases from the Benaki Museum in Athens, and
related X-ray spectra, obtained with a W-anode X-ray tube working at 30 kV
and 0.2 mA, and a Si-PIN detector.
55
5.4 Silver objects
Energy-dispersive X-ray fluorescence is not very suited to analyze
silver objects, because XRF is a surface technique, and silver is tipically
affected by surface enrichment processes. Further, a silver object can be
generally not be cleaned. For this reason X and gamma rays transmission
and scattering methods were applied together with XRF, both to silver
objects from Pompei and Ercolano 41. (at the Museo Archeologico in
Naples) and silver coins from the XVIII Century A.C., called baiocchi
42.
Compared results of transmission and XRF-results are presented in
Table 16.
Table 16 – Analysis of ancient roman silver objects from Pompei
Object and n.
R/C
XRF
Other
(%Ag)
(%A
eleme
g)
nts
Plate IV-25299
100
92
Plate IV-25313
99.5
91
Basket V-25343
100
94
Mug XIII-25290
100
93
Mug XIII-25291
98.5
95
Cup IV-25691
100
93
Pot III-125262
91
95
Cup IV-25373
100
87
Plate III-25350
94
93
Cu
Plate III-25351
93
92
Cu
Mug IV-116330
73
91
Cu
Mug IV-116332
77
91
Cu
Cup XVI-25565
94
73
Cu
Mug
XVII92
83
Cu
25578
Mug XVI-25301
88
84
Mug XVI-25300
99
79
Cu
Cup IV-110846
93
87
Cu,Pb
5.5 Ceramics
56
ACKNOWLEDGEMENTS
This work was partially supported by the Consiglio Nazionale delle
Ricerche, Programma Finalizzato "Beni Culturali”.
The authors are grateful to:
-Drs. P. Santopadre and M. Ioele of the “Istituto Centrale del Restauro”,
Rome
-the archaeologists of the Benaki Museum, Athens
-Dr. S. Bandera, Soprintendenza Beni Artistici e Storici, Milano
-Prof. P. Picozza and coworkers, Fondazione De Chirico, Rome
-Giovanni Moriggi.
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